So I have a question that my poor personal trainer is clueless on, having never trained a trans guy before... When figuring out my caloric intake, should I figure it as a male or female now that I am on T?  There is about a 200 calorie difference between one and the other when I calculate it using the formula I was given, with males taking in more calories.  So which should I try first--less or more?  I have been on T for not quite a month and am thin (125 pounds, 5'7" tall, small bone structure).  I am leaning toward going for more cause I REALLY want to gain, NOT lose.  But as someone who has spent his whole life eating fast food, this nutrition stuff has me lost!

If you're trying to gain muscle then start out with more.  Monitor your weight and body fat for about a month and if you find yourself gaining too much fat then reduce the calories a little.

If you are trying to gain muscle (bulking) then some fat gain is inevitable.  You can lose that later.  Just don't let it get out of hand and turn into a balloon.

Also, even with male T levels muscle gain isn't really very fast.  If your diet and program are optimal the best you can expect is between 1 and 2 pounds a month.  If your weight is going up faster than that then it's fat getting stored.

I would go with a little higher intake at your size (very similar to mine). I have struggled to gain weight/muscle/bulk despite my ravenous appetite while on T for the past 5 years.  I am a runner, which seems to keep me a little on the lean side.  Extra protein may help you build the muscle you crave.

Yes, protein is critical, but not too much protein.  At your current size you will need about 87 to 114 grams of protein per day.  Less than that will inhibit muscle gain.  More than that won't hurt you but won't speed up muscle gain either.

I agree with everything Deborah has said. Just going to chime in here too.

To my knowledge, the reason for the calorie difference between "male" and "female" in formulas is simply due to the fact that the average "male" has a higher lean body mass than a "woman". Lean body mass= muscle. Muscle requires more energy to maintain, therefore resulting in a higher caloric need.

Generally in a "bulking" phase, meaning  trying to gain weight/lean mass, you should aim to consume about 500 calories over your total daily energy expenditure. This would have you gaining about 1lb a week, which is usually the recommendation. As Deborah said, monitor your weight and make adjustments as necessary.
But remember your weight will fluctuate day to day, and even throughout the day. Try to only weigh yourself once a week, and always first thing in the morning before you eat or drink anything.
You WILL gain some bodyfat when eating at a caloric surplus, it's unavoidable. The amount of lean mass you also gain is entirely dependent upon the quality of your diet and how you're training. Diet is the most important factor however, so if you want to make quality gains, you're going to have to opt for less fast food.

Again, like Deborah said, gaining muscle takes a LONG time.. you will not see drastic physique changes in a month, two months, etc. and will likely be six months of consistency before results become noticeable.

Having a personal trainer is a good start, they can probably teach you the foundation. I also recommend just reading as much as you can, and there's a plethora of helpful videos on youtube as well. Plenty of free information out there on the internet... just don't believe anything that tells you that you need any sort of supplement to make muscle gains. You don't, not even protein powder, although it does help you achieve protein intake much easier.